Hybrid magnesium cement

Abstract

A magnesium oxychloride cement that has improved strength retention in wet conditions and durability testing, said cement incorporating: magnesium oxychloride hydrate cement, magnesium oxysulphate hydrate cement, a phosphate binder, and magnesium silicate hydrate cement, said magnesium silicate cement incorporating magnesium oxide and silica fume.

Description

HYBRID MAGNESIUM CEMENTTechnical Field

[0001] The invention relates to the field of magnesium cement manufacture. In particular, the invention relates to magnesium-based cements having improved durability properties with respect to: water resistance, including warm water (60°C) resistance; and corrosion resistance; and a method of making same.Background of the Invention

[0002] Magnesium oxychloride cement (MOC) in its most basic form is comprised of light burnt magnesia (magnesium oxide - MgO) and ‘brine’ (aqueous magnesium chloride - MgCh). It was invented by Stanislaus Sorel in 1867 and is also known as Sorel cement.

[0003] It has relatively high strength, fire resistance and acoustic performance compared to more typically used Portland cement. The past 20 years have seen an increased development of MOC-based lightweight building boards (MgO Boards) for use in the construction industry as structural flooring, and as internal and external cladding. This has been driven by the demand for products with high strength, non-combustibility and fire resistance.

[0004] However, known drawbacks in the use of MOC are a loss of strength following prolonged contact with water, along with the potential for causing corrosion of metal fixtures and fixings in contact with the cement due to the leaching of chloride ions. Much research has been undertaken to improve water resistance of MOC however its long-term durability in relation to water or moisture resistance has remained in question, which has limited its adoption in the construction industry.

[0005] The hydrophilic and breathable behaviour of MOC allows water to travel through the pores and capillary structures of the MOC, thereby breaking down its structural phases. In the presence of water, MOC structural phase 5 breaks into phase 3, with the formation of other reaction products, such as Brucite (Mg(OH)2) and chloride ions (Cl ), followed by a number of serious issues, such as leaching, efflorescence, metal corrosion and carbon sequestration, that eventually degrade the strength of the MOC.

[0006] Moreover, Brucite reacts with atmospheric carbon dioxide and carbon sequestration of MOC occurs with the formation of magnesium carbonate compounds and water. The free chloride ions travel to the surface via the porous structures andgenerate the electrolyte that causes corrosion of any metal in contact with MOC in a moisture-rich environment.

[0007] Research has been done to improve the water / moisture resistance of MOC and has achieved a certain level of improvement for normal temperature water. However, one of the most important properties of a quality building product is required to be meet the durability standard. A significant benchmark in relation to long-term durability of any lightweight building board is a standardised test whereby the board is subjected to immersion in water at 60 ± 2°C for 56 days ± 2 days, after which its retained strength should not be less than 75% of the strength prior to immersion. This test, commonly known as the ‘Warm Water Test’, is published in ISO 8336 2009-05-15 ‘Fibre-cement flat sheets - Product specification and test methods’ at Clause 5.6.11, and in AS / NZS 2908.2:2000 ‘Cellulose-cement products Part 2: Flat sheets’, Clause 6.4.

[0008] Until recently, no known MOC had achieved strength retention greater than 75% under the above described test. In patent publication WO 2022 / 032348, the present inventors have disclosed that a MOC formulation, containing a certain proportion of phosphoric acid, sodium mono-fluorophosphates (MFP) and fly ash, can achieve improved strength retention for both room temperature water and warm water tests.

[0009] However, the strength retention of this formulation can be subject to some fluctuation for different material properties and different mix proportions. It is therefore desirable to provide an improved formulation that achieves more stable performance when used as a building material.

[0010] Accordingly, it is an object of the invention to provide a further improved MOC formulation that ameliorates at least the issues discussed above, and potentially other problems associated with the prior art.Summary of the Invention

[0011] This invention comprises a modified magnesia cement formulation with admixtures introduced at specific and novel quantities to achieve high durability upon exposure to water, and specifically to meet the requirements of the 60°C Warm Water Test according to the building product standard requirements such as AS2908.2, thereby reducing the risk of water related degradation issues such as metal corrosion, leaching, efflorescence, carbon sequestration, etc. The formulation according to the invention is referred to herein as a hybrid magnesia cement (HBMC).

[0012] The HBMC according to the invention comprises two or more magnesia cements and other cement binders. Magnesium oxide is the main component of the formulation, further incorporating additional cements and binders. These are typically magnesium oxychloride hydrate cement (MOC) and magnesium oxysulphate cement (MOS), and a magnesium silicate hydrate system that acts as a gel binder in order to enhance bond strength, reduce porous surface area and reduce the formation of brucite in the cement matrix.

[0013] Said magnesium silicate hydrate may preferably be composed of MgO and silica fume in the approximate weight ratio of 40:60 and is preferably activated with a polymeric phosphate binder (such as sodium tripolyphosphate and / or sodium hexametaphosphate), added at less than 5% of total weight of MgO and silica fume in said magnesium silicate hydrate composition.

[0014] The cement according to the invention demonstrates a high level of strength retention under the room temperature and warm water tests. It has been shown that the presence of the magnesium silicate hydrate substitutes the area of the formation of phase 5 crystals, which improves the strength of the cement even in wet conditions.

[0015] According to a first aspect of the invention, there is provided a magnesium oxychloride cement that has improved strength retention in wet conditions, said cement incorporating: magnesium oxychloride hydrate, magnesium oxysulphate hydrate, a phosphate binder a magnesium salt brine, and magnesium silicate cement, said magnesium silicate cement incorporating magnesium oxide and silica fume.

[0016] Preferably, said magnesium oxychloride hydrate incorporates magnesium oxide and magnesium chloride brine, wherein the mass ratio of magnesium chloride brine to any other brine present in said cement is approximately 70:30.

[0017] Preferably, said magnesium oxysulphate hydrate incorporates magnesium oxide and magnesium sulphate brine, wherein the mass ratio of magnesium sulphate brine to any other brine present in said cement is approximately 30:70.

[0018] Preferably, said phosphate binder is an 85% concentration phosphoric acid solution and is present in said cement in a level equivalent to a 1 :1 molar ratio with 2% of the total mass of magnesium oxide in the cement.

[0019] Preferably, said magnesia cements (MOC & MOS) comprise between 85% and 95%, preferably 90%, of the total mass of magnesium oxide in said cement.

[0020] Preferably, said magnesium silicate cement system comprises between 7% and 9%, and preferably 8%, of the total mass of magnesium oxide in said cement. Preferably said magnesium silicate incorporates magnesium oxide and silica fume at a mass ratio of approximately 40:60.

[0021] Preferably, said cement further incorporates a phosphate additive, such as sodium hexametaphosphate or sodium tripolyphosphate, wherein said additive is incorporated at between 1.5% to 3.0% by mass relative to the combined mass of magnesium oxide and silica fume in the magnesium silicate cement system. Preferably the total inclusion of phosphates (comprising said phosphate binder and said phosphate additive) is between 0.2% and 0.5%, more preferably 0.3%, by mass of the cement.

[0022] Preferably, the cement of further incorporates other cementitious materials, including but not limited to fly ash and ground granulated blast-furnace slag. Preferably, the cement further incorporates one or more inorganic fibres selected from the group comprising: rice husk fibre, poly-vinyl acetate (PVA) fibre, chopped glass fibre strands, and AR glass fibre; and / or an organic fibre, for example rice husk.

[0023] In a particularly preferred embodiment, the cement has the following composition by mass:

[0024] 60% to 65% of total is magnesium oxychloride hydrate;

[0025] 25% - 30% of total is magnesium oxysulphate hydrate;

[0026] 0.50% to 1 .5% of total is MPC or magnesium phosphates; and

[0027] 3.0% - 8.0% of total is magnesium silicate hydrate gel.

[0028] Preferably, the cement further incorporates a course aggregate, for example aluminium oxide grains of between 1 mm and 5mm in diameter, and / or other inclusions, such as: natural crushed rock, granite chips or basalt based blue metals.

[0029] In another aspect of the invention, there is provided the use of magnesium silicate hydrate binder in a formulation of a magnesia cement to improve strength of said cement and / or improve retention of strength when exposed to water.

[0030] Now will be described, by way of a specific, non-limiting example, a preferred embodiment of the invention with reference to the figures.Brief Description of the Figures

[0031] Figure 1 a and 1 b are Scanning Electron Microscope (SEM) images of formation of cement paste of Hybrid Magnesia Cement according to the invention, where: 1 a and 1 b are the dry specimens after 28 days from the casting.

[0032] Figure 2a and 2b are SEM images of Hybrid Magnesia Cement according to the invention, where: 2a is a specimen without fillers; 2b is a specimen with fillers.

[0033] Figure 3a, 3b and 3c are SEM images of Needle-like Crystal of Hybrid Magnesia Cement under various test conditions, where: 3a is a control specimen in dry condition; 3b is a specimen under room temperature water (20°C - 30°C) for 28 days; and 3c is a specimen held under water (55+ / -5°C) for 28 days.

[0034] Figure 4a and 4b are X-ray diffraction analyses of Hybrid Magnesia Cement according to the invention, where: 4a is a dry specimen without fillers; 4b is a dry specimen with fillers.

[0035] Figure 5a, 5b, 5c and 5d are X-ray diffraction analyses of Hybrid Magnesia Cement specimens according to the invention, where: 5a was under room temperature water for 28 days; 5b was under room temperature water for 56 days; 5c was under 60°C warm water for 28 days; and 5d was under 60°C warm water for 56 days.Detailed Description of the Invention

[0036] The invention resides in a formulation of a magnesium oxide based cement that aims to improve durability of said magnesium oxide cement in terms of water resistance, especially with regard to the aforementioned 60°C warm water test, which is one of the durability requirements for cementitious flat sheet building products in accordance with AS2908-2 and ISO133688.

[0037] The invention may be described as a new combination of two or more magnesium cements and other binders. It is called the hybrid magnesia cement (HBMC) in this document.

[0038] In a preferred embodiment, said HBMC compromises a combination of magnesium oxychloride hydrate (MOC), magnesium oxysulphate hydrate (MOS), magnesium-silica cement, and phosphate binders.

[0039] In a preferred embodiment, MOC is used as the main binder due to its ability to produce ‘needle-like’ phase 5 crystals which provide a strong and stable structural phase at room temperature. Some proportion of MOC in the whole HBMC matrix is preferably combined with MOS in order to reduce the chlorine content of the matrix, as this chlorine may promote corrosion of any metals that contact the MOC in a moisture rich environment.

[0040] In some embodiments, the HBMC includes small parts of phosphate binders such as lead acid, phosphoric acid, magnesium phosphate, polymeric phosphate binders, etc., in order to form insoluble magnesium phosphate and accelerate the formation of cement phases by using as a catalyst. Phosphate binder ionizes in water and accelerates the exothermic reactions of the matrix. Further, the HBMC incorporates a magnesium silicate hydrate (M-S-H) system as a gel binder in order to: enhance bond strength, reduce porous surface area, and reduce formation of brucite in the cement matrix, thereby enhancing water resistance.

[0041] Said M-S-H gel binder system is composed of MgO and silica fume in the approximate weight ratio of 40:60 and is activated with a polymeric phosphate binder (such as sodium tripolyphosphate and sodium hexametaphosphate) incorporated at less than 5% of total weight of MgO and Silica fume in the component magnesium silicate cement composition.

[0042] As an example, in one embodiment the HBMC has the following composition:

[0043] 60-65% of total HBMC is MOC

[0044] 25% -30% of total HBMC is MOS

[0045] 0.50% to 1 .5% of total HBMC is MPC or magnesium phosphates

[0046] 3.0% - 8.0% of total HBMC is M-S-H system gel

[0047] In this description, strength retention in water resistance tests refers to ability to maintain structural strength when exposed to water at normal temperature water or warm water. Warm water resistance is a durability requirement for cementitious flat building products whereas room temperature water resistance is a general requirement for all building products in accordance with AS2908-2 and ISO133688.

[0048] Normal water temperature is typically between 16°C to 30°C and warm water is between 60±2 °C.

[0049] MOC typically has a porous structure that allows water ingress and travel throughout the cement matrix enhancing. In the present invention, the inventor has replaced some parts of the MOC, by comparison with known MOC formulations, with other binders to reduce porosity and to enhance mechanical properties of original cement.

[0050] In some embodiment, the total MgO powder content in HBMC matrix can be conceived as combination of four main components: 90% of the MgO powder present is used to form MOC and MOS, with and 70% and 30% of said MgO being used in MOC and MOS respectively. About 8%-9% of MgO powder in the cement overall is used in the M-S-H gel system and the remainder is used in formation of insoluble phosphates.

[0051] Inclusion of the MOS reduces the overall chlorine content in the matrix, thereby reducing the risk of contact steel corrosion in a wet environment. The MOC and MOS comprise the MgO powder with respective brines: magnesium chloride brine (brine 1 ) and magnesium sulphate brine (brine 2) with the weight proportion of brine 1 to brine 2 is 70:30 in this embodiment of the invention.

[0052] The salt concentration ranges of magnesium chloride brine (brine 1 ) and magnesium sulphate brine (brine 2) are 24.2% - 28.3% and 18% - 26% respectively. Brine temperatures used in the composition are within 20°C to 30°C.

[0053] Including said M-S-H system gel formation, preferably with fillers such as fly ash and ground blast black furnace slag (GBBFS), minimizes the porous surface and enhances the mechanical properties of the matrix. In a preferred formulation, the Magnesium Oxide to Silica Fume (SF) (MgO:Si2O) ratio by weight is approximately 40:60, and the total M-S-H binder mass is less than 10% of the whole HBMC matrix.

[0054] Hydrolysis of MgO and dissolution of SF influences the formation of M-S-H gel in the presence of water, either with or without additives such as polymeric phosphates (e.g. sodium hexametaphosphate (NHMP), trimetaphosphate or orthophosphate, etc.) The inventor has learnt that such additives have a plasticising effect and accelerate the formation of binding phase of M-S-H, but do not significantly alter the M-S-H structures.

[0055] In an example composition according to the invention, a 2M solution of sodium hexametaphosphate (NHMP) may be used at an inclusion level of 1.5 - 3.0% of the combined weight of MgO and SF.

[0056] However, in an alternative embodiment, the HBMC may incorporate with NHMP to accelerate formation of brucite and Mg2+ions to suppress the M-S-H gel formation.

[0057] To form insoluble magnesium phosphates and to accelerate the hydration process, a small proportion of the main MOC material is replaced with a phosphate binder. In this invention, phosphoric acid is preferable and the preferred MgO to Phosphate molar ratio is 1 :1. This enhances the water resistance of the cement by forming insoluble compounds, such as MPC or MgPOa.

[0058] In a preferred embodiment, M-S-H gel is produced by reacting 40:60 parts of magnesium oxide and silica fume (MgO / SF) with water which included sodium hexametaphosphates (NaPOs)6 as an additive to accelerate the formation of M-S-H gel, to improve rheology, strength and reduce the porosity of HBMC.

[0059] In this invention, the purity of Magnesia is typically in the range of 85% to 95%. The concentration of MgCh aqueous solution is 23% to 28%, whereas that of MgSCU aqueous solution is 24%-26%. The preferred molar ratios used in this invention are: MgO:MgCl2 (Mi) is between 6.0 to 9.5 and MgO:MgSC (M2) is between 8.0 to 10.0.

[0060] As phosphoric acid (H3PO4) enhances the formation of cement phases, it is preferred to match the proportion of pure NHMP to H3PO4 S0 that it is within 0.2 to 0.5 by weight, 0.3 is preferable.

[0061] It is further useful to incorporate in the HBMC fillers such as cementitious materials like fly ash, ground granulated blast-furnace slag (GGBS) or the like, to further enhance the mechanical properties and water resistance of the HBMC.

[0062] It is further useful to incorporate in the HBMC organic fillers, such as rice husk fibre, in order to achieve light weight and strengthen the matrix.

[0063] It is further useful to incorporate in the HBMC crushed coarse aggregate, such as basalt-based blue metal or granite, at a size of less than 20mm, preferably less than 10 mm for thinner board products.

[0064] The material specifications for the main components are given in Table 1 .Example - Method of Manufacturing HBMC

[0065] In the example below, a method of manufacturing HBMC according to the invention is presented. The materials used in this example are specified in Table 1 below, along with the relative proportions of the materials in the formulation. Cement admixture means in this example means the formulation of the cement per se, without fillers, aggregate and fibre reinforcements.

[0066] The equipment and techniques below, except where specifically noted, will be familiar to any manufacturer of MgO based cement products.Table 1 - Material Specification for Hybrid Magnesia Cement (HBMC)

[0067] The manufacturing procedure is as follows:

[0068] Step 1 : The magnesium chloride brine (brine 1 ) and magnesium sulphate brine (brine 2) with specified solid concentrations are prepared 24 hours ahead of making the HBMC composition. The brine temperature is between 22°C to 30°C.

[0069] Step 2: The hybrid brine is prepared by adding the brines and binders, according to the calculated proportion, into a mixer and is mixed for about 30 seconds to 60 seconds.

[0070] Step 3: The MgO and cementitious materials, fly ash, silica fume and slag are added into the alkaline aqueous solution prepared in Step 2 and blended until dispersed well.

[0071] Step 4: If required, the coarse aggregates and desired fillers (such as rice husk, exfoliated perlite vermiculite, etc.) are added and blended for about 60sec.

[0072] Step 5: If required, reinforcement fibres (such as PVA, Glass fibre, AR glass fibre, etc) are added and mixed for 60sec.

[0073] Step 6: The slurry is blended for a further 10 minutes to 15 minutes to achieve a homogeneous mix.

[0074] Step 7: The slurry is poured into a mould and vibrated for about 30sec on a vibrating table.

[0075] Step 8: The cast panels are placed in a first curing room (1 ) with the room temperature not less than 20°C and RH65% ± 5% for 24 hours.

[0076] Step 9: The products are demoulded (if necessary) and moved to a second curing room (2) with temperature set at 45°C±5°C with RH 70% ± 5% for 4 to 5 hours.

[0077] Step 10: The panels are allowed to cool down to room temperature.

[0078] The products can then proceed to post-production and / or packing and delivery to site. It is recommended that the products are usable for any test or structural loading purposes after 28 days from the casting, as per normal cement panels.

[0079] An alternative example composition for the magnesium oxide cement matrix according to the invention is shown in table 2 below.Table 2 - Example compositions for Hybrid Magnesia Cement (HBMC)

[0080] The mechanical properties of the HBMC panels according to the invention have been measured to be within the range of 32-60MPa for compressive strength and their flexural strength shows in the range of 9-24MPa, per tests performed in accordance with ASTM C0109M-16A and ASTM C348-18 respectively, as per table 3.

[0081] The direct tensile strength is about 3MPa for the HBMC matrix with 40MPa compressive strength. Variations can occur due to the specific fillers and fibres used in the matrix.Table 3 - Mechanical Properties for Hybrid Magnesia Cement (HBMC)

[0082] In table 4, the strength retention of the HBMC specimens are shown to be within 77%-94% of initial strength after between 1 day and 56 days under the room temperature water test.

[0083] However, the strength retention of the HBMC specimens under the warm water test in Table 4 shows a strength retention of within 99%-102% of initial dry strength. It means that the strength of the specimens shows little to no change, or may slightly increase, while under warm water for up to 56 days.Table 4 - Strength Retention of HBMC for water resistance testsComparison of XRD analysis

[0084] The HBMC according to the example prepared above has been assessed via XRD (X-ray diffraction) analysis, XRF (X-ray fluorescence) quantitative analysis and SEM (Scanning Electron microscopy) analysis.

[0085] Turning to the figures, Figure 1 a and 1 b are Scanning Electron Microscope (SEM) images of formation of cement paste of HBMC according to the invention, where: 1 a and 1 b are the dry specimens after 28 days from the casting. It will be seen that, unlike in other prior art magnesia cements, the formation of phase 5 crystals in dry HBMC specimens is obviously reduced and the length of the crystal is about 20pm-30pm in length.

[0086] Figure 2a and 2b are SEM images of HBMC according to the invention, where: 2a is a specimen without fillers; and 2b is a specimen with fillers. These show the formation of magnesia cement particles instead of crystal phases.

[0087] Figure 3a, 3b and 3c are SEM images of the needle-like crystal formation of HBMC under various test conditions, where: 3a is a control specimen in dry condition; 3bis a specimen under room temperature water (20°C - 30°C) for 28 days; and 3c is a specimen held under warm water (55+ / -5°C) for 28 days. It is possible to observe that the phase 5 crystals in Figure 3(a) are the shortest and fewest among the three, and Figure 3(c) demonstrates the longest and highest quantity of phase 5 crystals, followed by those in Figure 3(b). That means during the period under water or under warm water, the structural phase 5 crystals appear to have become stronger.

[0088] Figures 4a and 4b are X-ray diffraction analyses of HBMC according to the invention, where: 4a is a dry specimen without fillers; 4b is a dry specimen with fillers. As can be noted from the scan, the HBMC produces structural phase 5 crystals in accordance with the proportion of other ingredients. It is greater than 60% in the specimens without fillers and it is about 37.6% in the specimens incorporating fillers.

[0089] The inventors have also observed that the formation of K+ and Na+ feldspar minerals occurs regardless of the addition of aggregates and / or fillers. Adding blue metal (basalt based) aggregates increases the formation of feldspar minerals in HBMC as shown in Figures 4a and 4b. The hybrid magnesia cement forms microcline, albite and clinopyroxene based on the level of sodium, potassium, aluminium and silica contents. For example, the specimen without fillers forms about 4% of microcline but cement with other fillers such as aggregate, organic and inorganic fibres produces up to 13.4% and 24.4% albite and 2.8% clinopyroxene.

[0090] Figures 5a, 5b, 5c and 5d are X-ray diffraction analyses of Hybrid Magnesia Cement specimens according to the invention, where: 5a was held under room temperature water for 28 days; 5b was held under room temperature water for 56 days; 5c was held under 60°C warm water for 28 days; and 5d was held under 60°C warm water for 56 days.

[0091] The formation of microcline and albite, which have high hardness and high fire resistance, tends to increase after immersion under water, and moreso under warm water. As a result, the impact resistance, water resistance and fire resistance of HBMC specimens tends to increase.

[0092] It will be appreciated by those skilled in the art that the above described embodiment is merely one example of how the inventive concept can be implemented. It will be understood that other embodiments may be conceived that, while differing in their detail, nevertheless fall within the same inventive concept and represent the same invention.

Claims

Claims1 . A magnesium oxychloride cement having improved strength retention upon exposure to water at up to 70°C, said cement incorporating: magnesium oxychloride hydrate cement, magnesium oxysulphate hydrate cement, a phosphate binder, and magnesium silicate hydrate cement, said magnesium silicate cement incorporating magnesium oxide and silica fume.

2. The cement of claim 1 , wherein said magnesium oxychloride hydrate cement incorporates magnesium oxide and magnesium chloride brine.

3. The cement of claim 2, wherein the mass ratio of magnesium chloride brine to any other brine present in said cement is approximately 70:30.

4. The cement of claim 1 , wherein said magnesium oxysulphate hydrate cement incorporates magnesium oxide and magnesium sulphate brine.

5. The cement of claim 4, wherein the mass ratio of magnesium sulphate brine to any other brine present in said cement is approximately 30:70.

6. The cement of any preceding claim, wherein said phosphate binder is an 85% concentration phosphoric acid solution and is present in said cement in a level equivalent to a 1 :1 molar ratio with 2% of the total mass of magnesium oxide in the cement.

7. The cement of any one of claims 2 to 6, wherein said magnesium oxychloride hydrate cement and magnesium oxysulphate hydrate cement comprises between 85% and 95%, preferably 90%, of the total mass of magnesium oxide in said cement.

8. The cement of any preceding claim, wherein said magnesium silicate hydrate system comprises between 7% and 9%, preferably 8%, of the total mass of magnesium oxide in said cement.

9. The cement of claim 8, wherein said magnesium silicate hydrate system incorporates magnesium oxide and silica fume at a mass ratio of approximately 40:60.

10. The cement of claim 9, wherein further incorporating a phosphate additive, such as sodium hexametaphosphate or sodium tripolyphosphate, wherein said additive is incorporated at between 1 .5% to 3.0% by mass relative to the combined mass of magnesium oxide and silica fume.11 . The cement of any one of claims 6 to 10, wherein the total inclusion of phosphates (comprising said phosphate binder and said phosphate additive) is between 0.2% and 1 .0%, preferably 0.5% to 0.8%, by total mass of the cement.

12. The cement of any preceding claim, further incorporating other cementitious materials, including but not limited to fly ash and ground granulated blast-furnace slag.

13. The cement of any preceding claim, further incorporating one or more inorganic fibres selected from the group comprising: rice husk fibre, poly-vinyl acetate (PVA) fibre, chopped glass fibre strands, and AR glass fibre.

14. The cement of any preceding claim, wherein the cement has the following composition by mass:60% to 65% of total is magnesium oxychloride hydrate;25% - 30% of total is magnesium oxysulphate hydrate;0.50% to 1 .5% of total is MPC or magnesium phosphates; and3.0% - 8.0% of total is magnesium silicate hydrate gel.

15. The cement of any preceding claim, further incorporating an organic fibre, preferably rice husk.

16. The cement of any preceding claim, further incorporating a course aggregate, for example aluminium oxide grains of between 1 mm and 5mm in diameter.

17. The cement of any preceding claim, further incorporating one or more of: natural crushed rock; granite chips; basalt based blue metals.

18. The use of magnesium silicate hydrate binder in a formulation of a magnesium oxide cement to improve strength of said cement and / or improve strength retention when exposed to water having a temperature between room temperature and 70°C.

19. Use according to claim 18, wherein said binder is composed of magnesium oxide and silica fume in a mass ratio of approximately 40:60.

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